• Where two neurones meet, they do not actually come into physical contact with each other – a very small gap, known as the?synaptic cleft, separates them
• The ends of the two neurones, along with the synaptic cleft, form a?synapse

A synapse

Synaptic transmission – basic mechanism

• Electrical impulses cannot ‘jump’ across synapses
• When an electrical impulse arrives at the end of the axon on the?presynaptic neurone, chemical messengers called?neurotransmitters?are released from?vesicles?at the?presynaptic membrane
• The neurotransmitters?diffuse?across the?synaptic cleft?and?temporarily bind with receptor molecules?on the?postsynaptic membrane
• This?stimulates?the postsynaptic neurone to generate an electrical?impulse?that then travels down the?axon?of the?postsynaptic neurone
• The neurotransmitters are then?destroyed?or?recycled?to prevent continued stimulation of the second neurone, which could cause repeated impulses to be sent

The basic mechanism of synaptic transmission

Synaptic transmission – detailed mechanism

• There are over 40 different known neurotransmitters
• One of the key neurotransmitters used throughout the nervous system is?acetylcholine?(ACh)
• Synapses that use the neurotransmitter ACh are known as?cholinergic synapses
• The detailed process of synaptic transmission using ACh is as follows:
  • The arrival of an?action potential?at the?presynaptic membrane?causes?depolarisation?of the membrane
  • This?stimulates voltage-gated calcium ion channel proteins?to?open
  • Calcium ions?diffuse down an?electrochemical gradient?from the tissue fluid surrounding the synapse (high concentration of calcium ions) into the?cytoplasm?of the?presynaptic neurone?(low concentration of calcium ions)
  • This?stimulates ACh-containing vesicles to fuse with the presynaptic membrane, releasing ACh molecules into the synaptic cleft
  • The ACh molecules?diffuse?across the synaptic cleft and?temporarily bind to receptor proteins in the postsynaptic membrane
  • This causes a?conformational change?in the receptor proteins, which then?open,?allowing sodium ions to diffuse?down an electrochemical gradient into the cytoplasm of the postsynaptic neurone
  • The?sodium ions cause depolarisation of the postsynaptic membrane, re-starting the electrical impulse (that can now continue down the axon of the postsynaptic neurone)
  • To prevent the sodium ion channels staying permanently open and to stop permanent depolarisation of the postsynaptic membrane, the?ACh molecules are broken down and recycled
  • The enzyme?acetylcholinesterase?catalyses the?hydrolysis?of the ACh molecules into?acetate?and?choline
  • The?choline is absorbed back into the presynaptic membrane?and reacts with?acetyl coenzyme A?to form?ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives
• This entire sequence of events takes 5 – 10 ms

Synaptic transmission using acetylcholine (ACh)